Delayed coking involves thermal decomposition of petroleum residua (resids) to produce gas, liquid streams of various boiling ranges, and coke. Delayed coking of resids from heavy and heavy sour (high sulfur) crude oils is carried out primarily as a means of disposing of these low value resids by converting part of the resids to more valuable liquid and gaseous products, and leaving a solid coke product residue. Although the resulting coke product is generally thought of as a low value by-product, it may have some value, depending on its grade, as a fuel (fuel grade coke), electrodes for aluminum manufacture (anode grade coke), etc.
In a conventional (i.e., known to those skilled in the art of hydrocarbon thermal conversion) delayed coking process, the feedstock is rapidly heated in a fired heater or tubular furnace. The heated feedstock is then passed to a large steel vessel, commonly known as a coking drum that is maintained at conditions under which coking occurs, generally at temperatures above about 400° C. under super-atmospheric pressures. The feed (e.g., a heavy hydrocarbon such as resid) in the coker drum generates volatile components that are removed overhead and passed to a fractionator, ultimately leaving coke behind. When the first coker drum is full of coke, the heated feed is switched to a “sister” drum and hydrocarbon vapors are purged from the drum with steam. The drum is then quenched by first flowing steam through the drum and then by filling the drum with water to lower the temperature to less than about 100° C. after which the water is drained. The draining is usually done back through the inlet line. When the cooling and draining steps are complete, the drum is opened (i.e., the top and bottom heads are removed from the drum) and the coke is removed by drilling and/or cutting using, e.g., high velocity water jets.
Following coke removal, the top and bottom heads are re-attached to the first drum, and the process is repeated. Coking occurs cyclically in the sister drum as in the first drum, but with the coking in the second drum generally operated out of phase with the coking in the first drum. In other words, while feed is conducted to the first drum, the second drum is undergoing purge, quench, head removal, coke removal, or head re-attachment and preparation for feed admission. A plurality of drums can be used each cycling through the steps of the delayed coking process. Delayed coking processes have a characteristic cycle time, which is the time from the start of feed admission to a drum in a cycle to the point at which feed is admitted to the drum in the immediately succeeding cycle. In other words, the cycle time includes the time taken to conduct feed to a drum, coke the feed, purge the drum, quench the coke, remove the top and bottom heads, remove the coke, reattach the heads, and prepare the drum for feed admission.
In order to open the drum for coke drilling, the top head of the coker drum is loosened and moved away from the top of the drum. Similarly, the bottom head of the vessel is loosened and moved away from the vessel so that coke can be conducted out of the vessel and away from the process. The moving and replacing the removable top head and bottom head of the vessel cover is called heading and unheading (or deheading). Unheading has several associated risk factors, many arising from the risk of personnel and equipment exposure to rapid drum depressurization, steam and hot water.
Quenching, unheading, coke drilling, and coke removal add considerably to the cycle time (and throughput) of the conventional process. Thus, it would be desirable to be able to produce a free-flowing coke, in a coker drum, that would not require the expense and time associated with conventional coke removal, particularly the need to drill-out the coke. It would also be desirable to be able to safely remove such substantially free-flowing coke from the drum, preferably in a continuous process.